In an alternator of a battery charging system installed in a motor vehicle, especially, an alternator whose field winding is to be energized by a battery of the battery charging system, such a transient voltage may occur in the event of a battery disconnect due to engine vibration or the event of the high power loading of the battery is rapidly damped or removed from the battery charging system. In this case, a field winding becomes substantially opened to generate a transient voltage in stator windings during a period in which a field current drops by a comparatively long time constant of, for example, several hundreds milliseconds. This generation of such a rapid drop in the field winding will be referred to as “load damp”, and the transient voltage induced in the stator windings in the event of the load damp will be referred to as “load-damp voltage or load-damp surge voltage”, hereinafter.
In order to reduce an adverse influence on vehicle auxiliaries and/or components installed in the alternator, three patent publications have been proposed.
The first patent publication (U.S. Pat. No. 6,831,445B2 corresponding to Japanese Patent Application Publication No. 2003-174799) discloses means for applying a reverse bias voltage to the field winding after excitation thereof to thereby expedite attenuation in the field current.
The second patent publication (U.S. Pat. No. 7,119,519B2 corresponding to Japanese Patent Application Publication No. 2004-56881) discloses means for generating such a reverse bias voltage based on the battery installed in the motor vehicle.
The third patent publication (U.S. Pat. No. 4,516,066) discloses means for increasing such a bias reverse voltage by a series-connected batteries.
Referring to FIG. 12, there is illustrated a field-current control circuit disclosed in the third patent publication.
In FIG. 12, reference character 1030 represents high-side diodes (upper-arm diodes). The upper arm diodes 1030 works to supply a field current to the field winding 1014 from stator windings (not shown).
In the field-current control circuit, when a field-current adjusting transistor 1044 is turned off as need arises, a flywheel current (field current) flows through a field winding 1014 in a direction indicated by a solid arrow illustrated in FIG. 12; this flywheel current allows series-connected batteries 1025 and 1026 to be charged. The sum of the voltages of the batteries 1025 and 1026 is applied as a reverse bias voltage to a field winding 1014 so that the field winding is accelerated in attenuation. Specifically, a terminal A at which the field current is output from the field winding 1014 is reversely biased by the sum of the voltages of the series-connected batteries 1025 and 1026; this allows the field winding 1014 to be demagnetized.
The structure of the field-current control circuit illustrated in FIG. 12 may have the following problem. The problem will be described hereinafter with reference to FIGS. 13 and 14. FIG. 13 is a timing chart schematically illustrating the waveform of the field current of the field winding 1014 when the field-current control circuit operates in a normal mode in which no load-damp voltage is generated. FIG. 14 is a graph schematically illustrating a relationship between average field current and duty (duty cycle) of the field-current adjusting transistor 1044 when the field-current control circuit operates in the normal mode.
During an excitation accelerating period τ1 for which the field-current adjusting transistor 1044 has been in on state, the field current flows in a direction indicated by a dashed arrow illustrated in FIG. 12. In contrast, during an excitation damping period τ2 for which the field-current adjusting transistor 1044 has been in off state, the field current flows in the direction indicated by the solid arrow illustrated in FIG. 12 so as to flow through the series-connected batteries 1026 and 1025.
Specifically, when the field-current control circuit illustrated in FIG. 12 is used in such an alternator of a battery charging system, during the excitation accelerating period τ1, the terminal A substantially becomes a ground potential, and during the excitation damping period τ2, the terminal A substantially becomes the sum of the voltages of the series-connected batteries 1025 and 1026.
This unbalance causes an increasing waveform of the field current during the excitation accelerating period τ1 based on the field-current control circuit illustrated in FIG. 12 to be different from that of the field current during the excitation accelerating period τ1 based on a normal field-current control circuit. Similarly, the unbalance causes a damping waveform of the field current during the excitation damping period τ2 based on the field-current control circuit illustrated in FIG. 12 to be different from that of the field current during the excitation damping period τ2 based on a normal field-current control circuit.
This results that the field-current control circuit illustrated in FIG. 12 may cause the relationship between the duty cycle of the field-current adjusting transistor 1044 and the field current to become nonlinear (see FIG. 14).
The output of the alternator is designed to be controlled by controlling the duty cycle of the field-current adjusting transistor 1044 so as to adjust the field current. For this reason, the nonlinearity of the relationship between the duty cycle of the field-current adjusting transistor 1044 and the field current may make it difficult to stably adjust the output of the alternator.